The present invention relates generally to radio telemetry systems; and more particularly to improvements in method and apparatus for passively determining the distance from a ground station to a remote radio transmitting platform, such as a balloon-borne radiosonde.
Several balloon-borne radiosonde systems currently in use employ an active transponding system for determining wind speed and direction and height at a radiosonde in the upper air relative to a ground station. In addition, meteorological measurements of pressure, temperature and humidity at the sonde are also telemetered to the ground station on the same carrier radio frequency to enable complete evaluation of upper air conditions. The transponder system includes a stable oscillator and transmitter at the ground station for transmitting a sine-wave modulated range tone or signal on the carrier. Meteorological Sounding System (MSS) and National Oceanographic and Atmospheric Administration (NOAA) transponding radiosondes typically modulate a 400 to 406 MHz carrier band at 74978.13 Hz (nominally 75 kHz), which has an equivalent wavelength of 4,000 meters with each 360.degree. change in phase. The signals from the ground station are received in the radiosonde, demodulated and retransmitted on another radio carrier frequency, such as 1680 MHz, with the other meteorological measurements. The ground station receives the transmission, and separates and demodulates the range tone signal from the other data. A phase comparator determines the time lag of the outgoing sinewave to the return signal which is a measure of the time the signal took to reach the distant radiosonde and return to the ground station. The physical distance is a function of this time and the frequency of the modulated sine wave.
The radiosonde is launched from a precisely known position on the ground relative to the transmitting and receiving antenna, and the slant range distance is continuously computed from the moment of release. If the angle of elevation from the ground station to the sonde is known together with the total phase lag, the altitude and ground level distance of the sonde can also be computed using the sine and cosine relationships, respectively. The rate of change of ground distance will represent the wind speed, and the azimuth from the ground station will indicate the wind direction.
Although useful for their designed purposes, there are several drawbacks in current radiosonde transponder systems. For example, the initial cost and attendant upkeep of the ground-based transmitter are relatively high. Because of the inherently broad-band operation, only one radiosonde can be active at any one time because other radiosondes in the same geographical vicinity will often cause interference. With the large number of radiosondes currently in use, and the proliferation of other systems transmitting at or near the 403 MHz band, such interference has grown in proportion, reaching the point where many meteorological flights are lost or contain lapses and unreliable data. Being an expendable device, the radiosonde may contain a relatively simple and inexpensive receiver susceptible to sudden phase shifts due to swinging of the radiosonde from the balloon, and other atmospheric anomalies which may degrade the time lag measurements. In addition, since the ground station must transmit a ranging signal to the radiosonde, it cannot operate in a passive mode.